Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 5-100 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, for example in the form of a droplet, stream or cluster of material, with a laser beam (generated by a so-called “drive laser”).
For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include out-of-band radiation, high energy ions and debris, e.g., atoms and/or clumps/microdroplets of the target material.
These plasma formation by-products can potentially heat, damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input window. The heat, high energy ions and/or debris may be damaging to the optical elements in a number of ways, including coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at EUV wavelengths, corroding or eroding them and/or diffusing into them. Thus, it is typically desirable to minimize the amount of and/or the effect of plasma generated debris.
As indicated above, one technique to produce EUV light involves irradiating a target material. In this regard, CO2 lasers, e.g., outputting light at infra-red wavelengths, e.g. 9.3 μm or 10.6 μm, may present certain advantages as a drive laser irradiating a target material in an LPP process. This may be especially true for certain target materials, e.g., materials containing tin. For example, one advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power. Another advantage of CO2 drive lasers may include the ability of the relatively long wavelength light (for example, as compared to deep UV at 193 nm or Nd:YAG at 1.06 μm) to strongly reflect from relatively rough surfaces, such as surfaces that have become contaminated or coated with debris.
Another factor worthy of consideration is the difficulty associated with consistently and accurately hitting a series of relatively small, quickly moving droplets with a pulsed laser beam at relatively high repetition rates. For example, some high-volume EUV light sources may call for the irradiation of droplets having a diameter of about 5-50 μm and moving at a velocity of about 50-100 m/s, at repetition rates exceeding 30 kHz. Although one goal of LPP system design may be to provide a consistent, accurately timed series of target material droplets, it may be necessary to overcome shortcomings in droplet stability by quickly steering, focusing and in some cases changing the focal power of the irradiating laser beam. In addition, it may be desirable to accurately focus the beam to a relatively small focal spot with minimal aberrations. This may be especially challenging when the laser is a high power, multi-mode laser.
With the above in mind, applicants disclose systems and methods for drive laser beam delivery in an EUV light source.